Removal of organic materials (e.g., stripping of photoresist used for fabrication of semiconductor devices, cleaning of organic residues from surfaces of objects or structures, removal of polymers from crevices or grooves of structures or from other difficult regions of a structure, etc.) is one of various steps required in the production of semiconductor devices or other objects such as flat panel displays. For example, whenever photoresist or other organic masks are used, removal of organic material is typically necessary.
The removal of photoresist or other organic material is usually performed by either a dry or wet removal method. Wet removal techniques generally involve the use of specialized oxidizing solutions such as sulfuric acid and hydrogen peroxide solutions. Wet removal generally refers to the contact of a surface with a liquid chemical solution. For example, material is removed as an agitated liquid or spray passes over the surface. Dry etching refers to the contact of a surface with a gaseous plasma composition. Dry removal techniques generally remove organic material using oxygen plasmas, e.g., oxygen ash techniques, and possibly with hydrogen plasma assistance.
Removal of photoresist or other organic material from complex structures is typically done in an oxygen plasma ash tool or with a multiple step wet strip process, or combinations thereof. However, for example, as semiconductor device structure geometries and other structure geometries continue to get smaller, e.g., into the submicron range such as below 0.25 micron, neither conventional dry removal nor wet removal techniques provide adequate removal of organic material during the processing of such complex structures without damaging the structure being produced. For example, with device structures having critical dimensions below 0.25 micron geometries, conventional techniques may not be adequate for removal of organic material such as ultraviolet (UV) radiation hardened photoresist and/or sidewall deposited resist or residue, nor adequate for removal of organic material such as organic residue in difficult crevices or grooves of such device structures.
Various processing steps produce problems for the removal of organic material, e.g. photoresist. For example, surface hardening of photoresist due to reactive ion etching (REI) or ion implantation processes increases the difficulty in such removal. Further, for example, post formation bakes and UV curing steps may cause chemical changes in the photoresist that may cause difficulty in removal of the resists.
Both wet and dry organic removal techniques may not provide adequate removal when organic materials are present in complex structures, such as high aspect ratio openings, including submicron grooves, narrow crevices, etc. Wet stripping techniques do not appear to overcome such problems and generally result in incomplete photoresist removal. For example, both grooves and crevices render wet stripping solutions ineffective by limiting the solvent access to the organic material to be removed by reason of surface tension and capillary actions.
Dry techniques may also fail to completely remove such organic material in grooves and crevices, particularly due to the polymer formed on sidewalls of the resist. Dry plasma etching also appears to result in incomplete removal. For example, sidewall polymer formations may occur as a result of the interaction of released by-products of plasma etching with the sidewalls of the structure. Such polymers are not easily removed using ashing processes.
While wet etchants have many preferable characteristics as compared to dry etchants, dry etchants can be used in fabrication processes without the need to dry the structure being processed after a dry organic material removal step has been performed. The added step of drying the structure that is required when using a conventional wet etchant adds to the cost of fabrication. A lack of full process automation may also result from the added step of drying the structure. Another advantage of etching with a dry etchant is that it often decreases the safety hazards associated with wet removal solutions due to the relatively small amount of chemicals utilized in the dry etchant, e.g., environmentally advantageous.
Supercritical fluids have been used to remove organic residue from a variety of surfaces or extract substances from various materials. A gas is determined to be in a supercritical state (and is referred to as a supercritical fluid) when it is subjected to a combination of pressure and temperature so that its density approaches that of a liquid (i.e., the liquid and gas state coexist). For example, supercritical fluids have been used to clean contact lenses by etching residue from lense surfaces, as disclosed by Bawa et al. in PCT Application Publication Number WO 95/20476. Further, supercritical carbon dioxide (CO.sub.2), has also been used to remove exposed organic photoresist films, as disclosed by Nishikawa et al. in U.S. Pat. No. 4,944,837.
It is also known that organic material is removed rather well by certain oxidizing gases, in particular, sulfur trioxide. For example, as described in U.S. Pat. No. 5,037,506 to Gupta et al., entitled "Method of Stripping Layers of Organic Materials," issued Aug. 6, 1991, gaseous sulfur trioxide is used to remove various organic coatings, polymerized photoresist, and implant and deep-UV hardened photoresist layers during the manufacture of semiconductor or ceramic devices. Further, for example, as described in U.S. Pat. No. 4,778,536 to Grebinski, entitled "Sulfur Trioxide Vapor Phase Stripping" issued Oct. 18, 1988, water vapor is contacted with sulfur trioxide vapor adjacent the surface of an object to provide a hot mixture comprising sulfur trioxide, water, and sulfuric acid to remove photoresist